It would be great to set rules about what can be added to a cart based on item type and/or library. For example if a patron can only borrow 5 items of type book, it would be good to stop them from adding more than 5 books to their cart.
I know some libraries use the cart feature to collect and send information to the library. I believe this might be the use case the library had in mind here? If so, it's not as easy as it might seem. The number of items a user can check out, can depend on a lot of different factors. The maximum number of checkouts can depend on home library/holding library/the patron's library/the library they are checked out at. It can also depend on the item types of items. So it could change depending on the contents already added to the cart. Also, it could depend on how many the patron has already added to the cart. The cart is available to users before login, so you'd have to enforce login to enforce the limits or force the user to remove some before being able to send it. If the user wants to use the cart for placing holds or just to create a list of items for later, an enforced limit could pose a problem.